HTMLTableElement.insertRow()

<table id="TableA">
  <tr>
    <td>Old top row</td>
  </tr>
</table>
<script type="text/javascript">
  function addRow(tableID) {
    // Get a reference to the table
    var tableRef = document.getElementById(tableID);

    // Insert a row in the table at row index 0
    var newRow = tableRef.insertRow(0);

    // Insert a cell in the row at index 0
    var newCell = newRow.insertCell(0);

    // Append a text node to the cell
    var newText = document.createTextNode("New top row");
    newCell.appendChild(newText);
  }

  // Call addRow() with the ID of a table
  addRow("TableA");
</script>